Field of the Invention
The invention relates to a receiver circuit for a mobile radio receiver, in particular a cordless telephone, which is equipped with a controllable amplifier with a variable gain level. In addition, the invention relates to a method for processing a received signal in a mobile radio receiver.
As a result of the further development of mobile communications systems, new requirements are being made of the future terminals and their system components so that, inter alia, a more flexible range of services can be offered. In particular, a greater degree of flexibility, a lower level of power consumption, smaller dimensions and lower costs are required.
One possible way of fulfilling these requirements is for the analog signal processing means which are mainly used in contemporary mobile radio receivers to be largely replaced by digital signal processing means. For example, in recent years, the increase in the power of digital technology has made it possible to further process even relatively high frequency intermediate frequency signals in a digital fashion, enabling the proportion of the analog functions in the radio frequency section of the receiver to be significantly reduced. Generally, the increasing conversion of means for carrying out signal processing tasks into digital technology is leading to improved integration and flexibility of the system concept, improvement of the long-term stability and to a reduced number of tuning points when optimizing the receiver.
In this context, it is already known to dispense with a channel selection in the analog field and instead digitize the broadband intermediate frequency signal and perform the channel selection in the digital domain by way of digital filters.
However, the already mentioned system advantages of such a solution are also accompanied by disadvantages. When a broadband radio-frequency intermediate frequency signal is converted into the digital domain, a considerable degree of supplementary expenditure for the analog-digital converter is necessary. When digitizing a broadband intermediate-frequency signal, the converter must be capable of fulfilling high dynamic and bandwidth requirements.
In order to be able to fulfill the high bandwidth requirements, it has already become known to use analog-digital converters which operate according to the flash method. It is a disadvantage that that type of converter is very costly and expensive.
In order to reduce the dynamic requirements of the analog-digital converter (dynamic differences of up to 100 dB can occur in the reception signal), it has become known to implement an automatic gain control AGC in the analog section. The AGC ensures that the input signal of the analog-digital converter is always set to an optimum level. It is a disadvantage that the AGC also requires a considerable degree of implementation expenditure.
In an AGC, the signal strength estimation is carried out in the region of the analog signal processing means, i.e. in the signal path upstream of the analog-digital converter stage. A known modification of an AGC is the PGC (programmable gain control). In a PGC, the gain setting is derived from a digital control word which is output by a signal strength estimator which is arranged in the digital signal processing region.
A mobile radio converter module with the designation HSP50214B from HARRIS Semiconductor (Intersil) is known which has a PGC and which carries out channel selection in the region of the digital signal processing means (by means of digital filters). The PGC is constructed from a cascade composed of 36 individual amplifier stages, each with a gain level of 2 dB which can be individually connected into the circuit and disconnected from it. The estimation of the signal strength is carried out in the digital domain on the composite signal, i.e. before the channel selection.
A disadvantage with that module is especially the high implementation expenditure of the PGC. Further disadvantages are the likewise costly implementation of the external analog-digital converter stage and the digital filter arrangement.